A femto base station refers to a small mobile communication base station that access a mobile communication core network through a broadband network installed in a building, such as a home or office. Inside a building, the coverage of a macro base station may be limited by the outer wall of the building. In such an environment, a femto base station may be installed in the building to expand coverage and enhance call quality. Further, the femto base station may efficiently provide a high-band wired/wireless convergence service. The femto base station is connected to a femto network controller (FNC) through a broadband network, and the femto network controller (FNC) is connected to the mobile communication core network.
Femto base stations may be classified into i) CSG (Closed Subscriber Group) femto base stations and ii) open access femto base stations according to the policy of permitting access to the femto base station. The CSG femto base stations permit only approved users' user equipments to gain access. On the contrary, open access femto base stations provide services to any user equipments that approach a place within a transmission radius without any limitation.
Currently, 3GPP defines CSG IDs necessary for identifying whether a femto base station is a CSG femto base station and a femto base station white list that is a CSG ID list of femto base stations to which a user equipment supporting communication with a femto base station may gain access and which is stored in the USIM (Universal Subscriber Identity Module) of the user equipment. The corresponding femto base station periodically transmits a CSG ID to the user equipment through a control channel (CCH) to verify that the femto base station is a CSG femto base station.
Further, the femto base station is wiredly connected to a femto gateway through the broadband network, and the femto gateway is connected to an MME (Mobility Management Entity) of the mobile communication core network. 3GPP defines, in the network structure of a femto base station, i) a scheme in which a femto gateway controls the femto base station and ii) a scheme in which an MME of the core network controls the femto base station. In the specification, these are collectively described as a femto network controller (FNC).
In the existing femto base station structure, the femto base station is wiredly connected with a femto network controller (FNC) and generates a control channel for transmitting control information between the femto base station and the femto network controller (FNC) and a data channel for transmitting user data between the femto base station and the femto network controller (FNC). The femto network controller (FNC) performs control that allows femto base station registered through the control channel to be smoothly operated. Such control may include managing femto base station paging messages, solving interference between femto base stations, allocating operating frequency resources of the femto base station, and managing mobility of user equipments connected to the femto base station. The femto base station(s) may transmit and receive user data of users that gain access through the data channel connected to the femto network controller (FNC).
Data transmitted from a user equipment to a specific femto base station through an uplink may go through a base station and may be then transmitted over a downlink to another user equipment connected to another femto base station through a femto network controller (FNC).
That is, a femto base station performs communication through a control channel and a data channel established between a femto network controller (FNC) and the femto base station. Communication resources need to be efficiently managed and distributed so that multiple femto base stations maintain smooth communication with multiple user equipments.
In an existing mobile communication scheme, a macro base station MBS) assigns an available frequency resource block to a user equipment connected to the base station for communication with the user equipment. When installed, a macro base station is connected to a core network through a wired backbone network so that the base station may be installed as planned according to cell planning and the quality of service (QoS) may be guaranteed.
Accordingly, in the case that channel coding management and transmission power management are conducted to fit a channel condition for a radio channel between an access user equipment and the base station, the service quality (QoS) for communication with the user equipment may be ensured.
However, since a femto base station (as used herein, the “femto base station” includes a “mobile femto base station”, a “multiple access-type femto base station”, or a “mesh femto base station”) supports a small number of user equipments, e.g., five or less user equipments, and provides a small transmission radius, the femto base station needs to actively generate a control channel and a data channel according to the position of an access user equipment and the channel situation and efficiently process communication data of the access user equipment.
FIG. 1 illustrates an example where multiple CSG femto base stations are connected to each other through a femto network controller (FNC) in a single macro base station.
Referring to FIG. 1, the CSG femto base station different from each other are positioned adjacent to each other in the transmission radius of the macro base station. The first femto base station 110, the second femto base station 120, and the third femto base station 130 correspond to CSG1, the fourth femto base station 140 and the fifth femto base station 150 correspond to CSG2, and the sixth femto base station 160 and the seventh femto base station 170 correspond to CSG3. The femto base stations are all connected to the femto network controller 180. A user equipment 190 is positioned in an area where the first femto base station 110, the third femto base station 130, and the fourth femto base station 140 overlap in terms of transmission radius.
Here, the user equipment 190 is assumed to be a dual mode user equipment that may gain access to multiple base stations at the same time. The dual mode user equipment may be a user equipment that may simultaneously use various radio access schemes, a user equipment that supports the CoMP (Cooperative Multipoint Transmission) technology currently defined in 3GPP LTE, IEEE 802.16m, which enables a single user equipment to perform simultaneous communication with multiple base stations, or a user equipment that supports multiple antenna technologies or smart antenna technologies.
When according the white list of the user equipment 190, the user equipment 190 can access both CSG1 and CSG2, the user equipment 190 may connect to all of the first femto base station 110, the third femto base station 130, and the fourth femto base station 140 at the present position (which is marked in solid lines) to thereby conduct communication.
However, since the femto base stations to which the user equipment 190 is connected are close to each other and thus transmission radii thereof overlap, interference may occur.
Accordingly, in such case, it is needed to adjust and manage interference and traffic between the user equipment and the femto base station with which the user equipment is communicating. That is, the cooperation between the adjacent femto base stations to which the user equipment is simultaneously connected is required so as to remove interference and process user equipment traffic.
However, the existing scheme, which performs such cooperation through the femto network controller (FNC) and uses a wired broadband network that may not guarantee QoS, has difficulty in adaptively performing cooperation between femto base stations. Moreover, when multiple user equipments connected to an adjacent femto base station attempt to communicate with each other, a method for quick data transmission without passing through a wired network is needed.
That is, unlike the open access femto base station, the CSG femto base station permits only limited types of user equipments to gain access, and thus, the cooperation between femto base stations with different CSGs is limited. In FIG. 1, the femto base stations corresponding to CSG1, the femto base stations corresponding to CSG2, and the femto base stations corresponding to CSG3 have difficulty in mutual cooperation due to differences in CSG values. Inter-CSG femto base station cooperation may be restrictively achieved between femto base stations having the same CSG, which permit access in the same user group.
Accordingly, there is a need for a method of being able to perform communication between femto base stations without any CSG limitation to the femto base stations for smooth cooperation between the femto base stations.